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If You Drain Them, Floods Will Come

Given how saturated the soil was last fall, coupled with record snowfall throughout the Canadian Prairies, it’s no surprise to witness the unprecedented flooding that has occurred along the Assiniboine River and its tributaries this year. However, this situation is much worse because of wetland drainage across the Prairies.

Wetland drainage increases the probability of flooding and associated damages in two ways. First, when wetlands are drained, the surrounding watershed’s ability to store water is reduced or completely eliminated. Wetlands hold and slowly release water, thereby reducing and delaying peak flows of water, which helps to decrease the impacts of severe events and extreme weather conditions. Secondly, the ditches that are built to drain wetlands not only drain water from the wetlands themselves, they also drain the lands that surround each isolated wetland. On average, for every acre of wetland that is drained, four additional acres of surrounding lands are also drained. This greatly increases the amount of water moving downstream, as well as the speed at which that water travels.

Furthermore, wetland drainage causes a tremendous amount of nutrients (such as phosphorus) to be leached from the land and moved to downstream areas, especially during floods. This nutrient loading causes many of the problems that we see in our lakes and rivers today.

There is a growing body of scientific evidence that links wetland drainage to increased flood damage. For example, when a study in Texas compared areas that experienced similar rainfall, it found that flood damages were higher in areas where wetlands were destroyed.

The authors concluded that, “Disrupting the natural hydrological system can exacerbate flooding or create flood problems in areas not originally considered vulnerable to this hazard.” In other words, when we damage or remove wetlands, we put more areas at risk from floods.

Some areas in the Prairies have lost more than 90 per cent of the wetlands they contained prior to European settlement. This level of wetland loss has reduced the landscape’s flood storage capacity by approximately 700 million cubic metres; that’s roughly seven times the flood storage capacity of the Shellmouth Reservoir near the Manitoba-Saskatchewan border. As a result, these impaired areas are releasing additional water to downstream areas, so that flood mitigation infrastructure (such as reservoirs, diversions and dikes) is under unnecessary stress and is working beyond capacity.

Two years of wetland drainage in southwest Manitoba has reduced surface water storage by 9.2 million cubic metres; that’s more than the volume of water that flowed through the breach at Hoop and Holler Bend in Manitoba, which was cut to relieve pressure on the dikes of the Assiniboine River.

In times like these, our first reaction is usually to improve infrastructure and disaster response plans to prepare for the next flood. However, if we allow wetland drainage to continue, we will only increase the unpredictable nature of our Prairie streams and rivers, which will require further investments in flood mitigation and planning, all of which could be ineffective or possibly even wasted. The message is clear: wetland drainage must stop if we ever hope to effectively manage flood risks and save our lakes.

Pascal Badiou is a research scientist with Ducks Unlimited Canada

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